If you are not watching the tv it would have to be a more efficient heater than the heater itself since a significant part of the energy is used to create the sound and display. I doubt that is the case so leaving it on would be "wasting" energy.

But the light and sound produced by the TV quickly become something else (light and sound don't stay around when your turn the TV off). Isn't this light and sound becoming heat? If not, what are they transforming into?

You mean from the power plant to my house? Ok, I'm not taking that into account (I could argue it's heating the outdoors, but that's really pointless). Apart from that, the energy that reaches my house would be 100% useful, I suppose.

Well there's a 6-7% hit in transmission and distribution that would be heating the outdoors which I think should be taken into account. Unfortunately that's really the only part of this problem I can speak on meaningfully.